1. Field of the Invention
The present invention relates to a semiconductor laser diode module capable of stabilizing the light intensity and wavelength of a semiconductor laser diode.
2. Description of the Related Art
Recently, semiconductor laser diodes have been used as light sources in high speed, long-distance and large capacity optical fiber communication systems. Particularly, for 10 km or more long-distance optical fiber communication systems, in order to suppress the effect of the wavelength dispersion, use is made of a single-axis mode semiconductor laser diode such as a distributed feedback (DFB)-type semiconductor laser diode which oscillates at a single wavelength.
In optical communications, it is important for the light intensity and wavelength of a light source to be constant. Generally, the light intensity and wavelength of a semiconductor laser diode are changed by the drive current and temperature, respectively, thereof. However, as the semiconductor laser diode deteriorates with time, the drive current is increased so as to slightly change the temperature of the semiconductor laser diode, so that the wavelength is also changed.
The above-mentioned slight wavelength change has hardly affected the conventional optical communication systems, however, this is fatal in dense wavelength division multiplexing (DWDM) optical communication systems where the pitch of wavelengths, i.e., the pitch of frequencies is very small, or 100 GHz or 50 GHz. In this case, the stability of wavelength is required to be  less than xc2x150 pm. Therefore, light intensity- and wavelength-highly stabilized semiconductor laser diodes are required for such optical communication systems.
In a first prior art semiconductor laser diode module (see: JP-A-1-209546), an optical branching unit including a gradually-thickness-changed bandpass filter is externally provided to control the light intensity and wavelength of the semiconductor laser diode. This will be explained later in detail.
In the above-described first prior art semiconductor laser diode module, however, since the optical branching unit is externally provided, the entire apparatus including the semiconductor laser diode module is increased in size. Also, since the gradually-thickness-changed bandpass filter is expensive, the manufacturing cost is increased.
In a second prior art semiconductor laser diode module (see: JP-A-4-157780), an optical branching unit including a gradually-slope-changed bandpass filter and transmission/reflected light detectors is provided within the semiconductor laser diode module. As a result, the entire apparatus including the semiconductor laser diode module is decreased in size. This also will be explained later in detail.
In the above-described second prior art semiconductor laser diode module, however, since the slope of the bandpass filter is gradually changed by a sloping mechanism, the control of slope of the bandpass filter is complex. Also, since use is made of a rear-facet light beam to control the light intensity and wavelength, the second prior art semiconductor laser module cannot be applied to a face emitting semiconductor laser diode or a wavelength tunable semiconductor laser diode array, and also, the availability of light beams is decreased.
In a third prior art semiconductor laser diode module (see: JP-A-10-79723), an optical branching unit including a gradually-slope-changed bandpass filter and two transmission light detectors is provided within the semiconductor laser diode module. As a result, the entire apparatus including the semiconductor laser diode module is decreased in size. This also will be explained later in detail.
Even in the above-described third prior art semiconductor laser diode module, however, since the slope of the bandpass filter is gradually changed by a sloping mechanism, the control of slope of the bandpass filter is complex. Also, since use is made of a rear-facet light beam to control the light intensity and wavelength, the third prior art semiconductor laser module cannot be applied to a face emitting semiconductor laser diode or a wavelength tunable semiconductor laser diode array, and also, the availability of light beams is decreased.
In a fourth prior art semiconductor laser diode module (see: JP-A-2001-257419), an optical branching unit including a bandpass filter, a transmission light detector and a direct light detector is provided within the semiconductor laser diode module. As a result, the entire apparatus including the semiconductor laser diode module is also decreased in size. This also will be explained later in detail.
Even in the above-described fourth prior art semiconductor laser diode module, however, since use is made of a rear-facet light beam to control the light intensity and wavelength, the fourth prior art semiconductor laser module cannot be applied to a face emitting semiconductor laser diode or a wavelength tunable type semiconductor laser diode array, and also, the availability of light beams is decreased.
In a fifth prior art semiconductor laser diode module (see: JP-A-9-219554), an optical branching unit including a coupler (beam splitter), two bandpass filters and two light detectors is provided within the semiconductor laser diode module. As a result, the entire apparatus including the semiconductor laser diode module is decreased in size. This also will be explained later in detail.
In the above-described fifth prior art semiconductor laser diode module, however, since the beam splitter is incorporated thereinto, the size of the module is increased. Also, since the two bandpass filters are provided, the manufacturing cost of the module is increased. Further, since use is made of a rear-facet light beam to control the light intensity and wavelength, the fifth prior art semiconductor laser module cannot be applied to a face emitting semiconductor laser diode or a wavelength tunable semiconductor laser diode array, and also, the availability of light beams is decreased.
In a sixth prior art semiconductor laser diode module (see: JP-A-9-121070), an optical branching unit including a coupler (beam splitter), a bandpass filter and two light detectors as well as a coupler (beam splitter) for an optical fiber is provided within the semiconductor laser diode module. As a result, the entire apparatus including the semiconductor laser diode module is decreased in size. This also will be explained later in detail.
In the above-described sixth prior art semiconductor laser diode module, however, since the two beam splitters are incorporated thereinto, the size of the module is increased.
In a seventh prior art semiconductor laser diode module (see: Y. Kai et al., xe2x80x9c32-wavelength Tunable LD Module Built-in Multi-wavelength Lockerxe2x80x9d, Communications Society Meeting of IEICE, p. 397, 2000), the two beam splitters of the sixth prior art semiconductor laser diode module are replaced by a 3-way prism-type beam splitter. This also will be explained later in detail.
In the above-described seventh semiconductor laser diode module, however, since the prism-type beam splitter is incorporated thereinto, the size of the semiconductor laser diode module is still increased.
It is an object of the present invention to provide a compact semiconductor laser diode module without beam splitters.
According to the present invention, in a semiconductor laser diode module including a semiconductor laser diode having a front facet for emitting a light beam, a collimating lens for receiving the light beam to generate a collimated light beam and a coupling lens for receiving the collimated light beam and converging the collimated light beam to an optical fiber, a bandpass filter is provided for receiving a first part of the collimated light beam, and a light detector is provided to have a first portion for receiving the first part of the collimated light beam through the bandpass filter and a second portion for directly receiving a second part of the collimated light beam. Thus, a wavelength of the semiconductor laser diode is controlled in accordance with an output signal of the first portion of the light detector, and a light intensity of the semiconductor laser diode is controlled in accordance with an output signal of the second portion of the light detector.